As a rolling bearing device for a wheel which is used for rotatably supporting a wheel of an automobile with respect to a suspension, there has been known a so-called hub unit including a tubular fixed bearing ring which is fixed to a vehicle side, a rotary bearing ring which is rotatably passed through this fixed bearing ring, and a rolling element which is disposed between these bearing rings so as to freely roll. In this hub unit, a plurality of flange portions to be fixed to the suspension are formed radially on an outer peripheral surface of the fixed bearing ring at its one end side in an axial direction, and at the same time, a plurality of flange portions to which a tire wheel and a brake disk of the wheel are to be attached are formed radially on an outer peripheral surface of the rotary bearing ring at the other end side in the axial direction. Moreover, a spigot joint portion in an annular shape projecting in the axial direction is formed at the other end of the rotary bearing ring in the axial direction.
According to request for reducing a weight of a vehicle body, in the above described hub unit too, it is desired to reduce a weight of the hub unit as much as possible, while securing predetermined strength and rigidity, similarly to the other members constituting the vehicle body.
For achieving the weight reduction, it has been conducted to form a circular recess in an end part of the rotary bearing ring at a side where the spigot joint portion is formed. Moreover, as for a shape of the flange portions, it has been conducted to leave only those areas surrounding bolt holes, and to cut away the other areas.
Further, it has been proposed to achieve the weight reduction while securing the strength, by modifying the shape of the aforesaid recess in the end part at the side where the spigot joint portion is formed (Reference should be made to Japanese Patent Publication No. JP-A-2001-47805, for example).
In the hub unit for supporting a wheel disclosed in the Japanese Patent Publication No. JP-A-2001-47805, the recess is formed in a bowl-like shape, and a depth of this recess is set to be 0.3 to 0.8 times as large as a diameter of the recess at its open end. An inner surface of the recess has a sectional shape of a circular arc in at least a part, and its curving direction is not varied in midway over the entire length.
There has been proposed another rolling bearing device for a wheel in which the aforesaid flange is extended radially outwardly in a radial direction from the rotary bearing ring, and formed with a plurality of arm portions formed with through holes to which hub bolts for securing the brake and the wheel of the vehicle are to be fixed (Reference should be made to Japanese Patent Publication No. JP-A-2003-94905, for example)
In the rolling bearing device for a wheel of this type, the brake rotor is fixed to the flange, in a state where the brake rotor is abutted against outer side surfaces of a plurality of the arm portions which are directed to an outward side of the vehicle, and clamped by the hub bolts and hub nuts to be screwed with these hub bolts.